The durability of a metal product is often determined by the fatigue strength of the product. In order to increase the fatigue strength, a general practice is to increase the design cross-section to thereby reduce stress. However, various other treatment methods may be employed to improve fatigue characteristics.
Treatment methods for improving fatigue characteristics may be broadly divided into two types. One type includes those methods, such as grinding and TIG dressing, which aim to reduce stress concentration by modifying the shape of the parts likely to be affected by fatigue. The other type includes those methods, such as hammer peening, needle peening, shot peening, welding using low temperature transformation material, etc., which aim to reduce the effective range of repeated stress by imparting compressive residual stress to the parts likely to be affected by fatigue. The above-mentioned hammer peening is considered to have both effects, that is, reduction of stress concentration and introduction of compressive residual stress.
Among the above-described treatment methods for improving fatigue characteristics, the methods that aim to reduce stress concentration have an effect that is evidently visible to the eye. In practice, however, when there is a minute flaw in the spot likely to be affected by fatigue, it may degrade the fatigue strength. Therefore, in order to be effective, the grinding treatment requires high skill and the execution of the treatment work may require much time. These factors result in a considerable increase in the cost.
Also, with respect to TIG dressing, highly skilled workers are required for the execution of the treatment work. As heat is applied to the treated site, in the case where this method is applied to repair a bridge beam, traffic must be stopped while the treatment work is being executed in order to avoid the occurrence of hot cracking of welding material due to stress variation. This is also a considerable cost factor.
On the other hand, in those methods which aim to introduce compressive residual stress, the compressive residual stress is not visible to the naked eye, and the effect of the treatment is difficult to measure and cannot be readily assured by inspection. Thus, in view of quality control, this type of treatment is usually not adopted unless an engineer competent in diagnosis and judgement can attend to the treatment work at the site.
In case of hammer peening, large plastic deformation can be imparted to the treated part, and since a large treatment mark is left, treatment can be identified after the work. A surface flaw that is produced at the time of treatment, however, may bring about stress concentration and result in degradation of fatigue strength. In addition, as workability is poor due to the large reaction produced when the plastic deformation is imparted, fine control is difficult and quality control becomes very difficult.
Especially when the treatment method for improving fatigue characteristics by introducing compressive residual stress as described above is applied, if there are cracks of 1 mm or less in length, which represent an early stage of occurrence of fatigue cracks and which cannot be detected by current examination methods such as liquid penetrant examination, magnetic particle examination, eddy current examination, or the like, application of the above-described treatment method for improving fatigue life in the presence of such cracks cannot stop the propagation of the cracks. Thus, it is considered that the introduction of compressive residual stress has little or no effect in improving fatigue life.
Also in the case of introducing compressive residual stress to a toe portion by welding using low temperature transformation material, although this treatment has large effect in high strength steel, it has little or no effect in low strength steel. In addition, as heat is applied in welding, it has the same problem in execution of work as in TIG dressing and is difficult to use in practice. Also, as in other treatment methods, effect of the introduced compressive residual stress is difficult to measure.
As has been described above, in the case of treatment methods for improving fatigue characteristics that aim to reduce stress concentration, there are problems mainly in efficiency of work execution and the skill required of workers. On the other hand, in the case of treatment methods for improving fatigue characteristics that introduce compressive residual stress, there are problems that effect of the treatment cannot be measured and quality control is not possible. Therefore, these treatment methods for improving fatigue characteristics are generally difficult to use.